A wide variety of medical devices are made from thermoplastic polymers. Medical devices must be manufactured with greater care than general consumer products especially when inserted into the body or brought into contact with a wound or lesion. In the area of treatment devices, such as catheters, manufacturers must take great care to assure that the devices perform with an extremely high degree of reliability. At the same time there is a need to develop materials and improve processing techniques to obtain improvements in desirable properties such as tensile strength, flexibility, puncture resistance, and softness.
For many years it has been recognized that polymer molecular weight should be high in order to optimize desired strength properties. However, maximizing molecular weight increases melt viscosity at a given temperature. Moreover, the molecular weight of thermoplastic polymers typically degrades in the melt, and as melt temperature increases the reaction rates of these degradative reactions increase, especially polyesters and polyamides. Consequently there are practical limits on the molecular weights which can be used to manufacture articles by processes, such as extrusion and many molding processes, which employ a polymer melting step.
Levy, U.S. Pat. No. 4,490,421, describes use of PET of high molecular weight (intrinsic viscosity is 1.0 or higher) to produce a balloon for a medical catheter. The patent notes that the intrinsic viscosity may decrease during processing into balloons. Such a decrease is believed to be related to polymer degradation caused by extrusion temperature, moisture and the time the resin is held in the melt.
Commonly assigned copending U.S. application Ser. No. 10/055,747, filed Jan. 23, 2002, describes medical devices, such as catheters and high strength balloons used thereon, which are formed from melt compositions of thermoplastic polymers and a chain extending additive. In this process the chain extending additive is reacted in the melt stage, with the consequence that melt viscosity is increased.
U.S. Pat. No. 5,250,069, Nobuyoshi et al, describes catheter balloons made of crosslinked ethylene-vinyl acetate copolymer, by means extruding or injection molding a tube of ethylene-vinyl acetate copolymer, crosslinking the copolymer in tube form by exposure to radiation from an electron beam or a gamma-radiation source, and then blow-forming the balloon from the tube of crosslinked copolymer. A similar crosslinking method has been used with commercial catheter balloons made from polyolefin-ionomer resins such as SURLYN® polymers. For many materials, however, exposure to such high energy radiation causes polymer degradation, rather than crosslinking. Consequently, radiation crosslinking is not generally suitable as a method of increasing molecular weight in the solid state.
In preparation of commercial condensation polymers it has been known to extend molecular weight of precursor polymers, initially obtained from melt, solution or dispersion phase polymerization reactions by solid state polymerization techniques. Examples of such processes are described in U.S. Pat. No. 6,187,895, Varadarajan et al, (solid state polymerization technique for polycarbonates and polyesters); U.S. Pat. No. 6,365,702, Hait et al (polyestercarbonates produced in solid state polymerization reaction with partially crystalline polycarbonate precursor); U.S. Pat. No. 5,955,569, Dujari et al, (extended polyamide produced by heating catalyzed precursor polyamide in solid state); U.S. Pat. No. 4,755,590, Kabánek et al, (solid state postpolymerization process for ε-caprolactam based polyamides in granule form); and U.S. Pat. No. 6,239,200, Kao et al, (solid state polymerization of polyester with hindered aromatic phosphate additive). Heretofore it has not been known to extend the molecular weight of a formed device article by such a process.